Using a point light source, such as a light-emitting diode (LED), to create a planar, uniformly emitting illuminating surface is difficult. Complex optical structures are required to distribute the light emitted from the LED evenly over the entire illuminating surface. An example of such a structure is a light guide that receives point-source light on an edge of the guide and distributes the light uniformly over a surface of the guide. As shown in FIG. 1, an edge-illuminated structure 100 may use a side-emitting point light source 102 that transmits light 104 to an edge 106 of a light guide 108. The light guide 108 distributes the transmitted light 104 to a top surface 110. The light source 102 is separate from the light guide 108.
The number of light sources that may illuminate the structure is limited, however, by the lengths of the light-guide edges and the dimensions of the light sources. As the surface area of the guide increases, more light sources than can physically fit on the light-guide edges may be required to maintain a constant illumination on the surface of the guide, ultimately setting an upper bound on the surface area. Moreover, an edge-illuminated light guide requires side-emitting, pre-packaged light sources, thereby limiting the number and types of light sources that may be utilized. Further, the structure required to couple light from a side-emitting light source into an edge of the light guide may impede miniaturization of the planar illumination system.
A planar illumination surface constructed from a plurality of light guides may exhibit non-uniform light intensity at the borders between adjacent light guides, or “stitches.” For example, the edges of the light guides may be non-uniform, allowing non-light-emitting gaps to form between the light guides. In addition, the direction of propagation of the light in two adjacent light guides may be different, creating a non-uniform pattern of light emission at the border. Finally, tiles that overlap one another may allow stray light to escape in the overlapping area. Clearly, a need exists for a planar illumination surface that is assembled from a plurality of light-guide elements and that emits a uniform light.